1. Field of the Invention
This invention relates to printing devices and in particular to printing devices having a print element the characters of which are selectively engaged by a hammer mechanism for high speed printing of characters on paper.
2. Description of the Prior Art
In current state-of-the-art serial impact printing, 30 to 55 characters per second is considered high speed. In many of such high speed printing devices a rotatable curved platen is used. As a result it has been generally concluded that in order to print on a paper placed against the curved platen, and obtain characters of uniform clarity and density, the type character must have a curved surface conforming to the curved surface of the platen.
It is common for a high speed printing device to employ a print wheel. These wheels are typically thin discs of plastic or metal, having slots cut radially around the circumference so as to provide a plurality of flexible spokes. The print characters are located on one side of the disc and towards the radially outer end of the spokes.
When made of plastic, the print wheel is generally injection molded. The mold is such as to provide the surface of the type characters with a curvature corresponding to the curvature of the platen. Printing is accomplished by striking the rear of the spoke, upon which the type character is located, with a hammer. Repeated hammering causes such plastic characters to deteriorate rapidly in print quality due to wear and deformation of the plastic print character.
It is therefore an object of the present invention to effect a durable print wheel which is both simple and economical to produce, and suitable for use in high speed printers which employ a platen which may not be flat.
Metal is the desirable material to use in making print wheels because of its durability. If a print wheel were to be made of metal, it was believed that the characters would have to be curved to conform to the surface of the platen. It is difficult to produce metal type characters (for high speed printing devices) having curved surfaces. An approach which has not proved successful was to etch flat surfaced print characters and then grind the curvature into the characters. A number of organizations have attempted to utilize techniques of this nature, but the expense and difficulty are so great that the efforts have terminated in failure.